Tire uniformity machines are well known in the art. In general, these machines test characteristics of a tire by placing the tire on a chuck and contacting it with a rotating load wheel that is equipped with load cells to measure characteristics of the tire. Typically, the load wheel is in a retracted position while the tire is being conveyed into the machine and mounted on the chuck. In this retracted position, the load wheel is moved laterally outward relative to the chuck assembly until the load wheel is more or less outside of the machine proper. Once the tire has been chucked, the load wheel is driven inwardly to contact the tire. The position of the load wheel relative to the tire is important in terms of applying a specified load to the tire and ensuring accurate test results.
In one known system, the motion of the load wheel is controlled by a D.C. motor via a 15:1 gear box reduction and a 54 T-23T chain-sprocket drive increase with a resulting reduction of 6.39:1 to a 0.25 inch pitch ball screw. The maximum speed of such a system is about 1.1 inches per second. Loading is accomplished by using a 4-quadrant D.C. drive that controls the velocity of the load wheel carriage. Using the velocity controlled method, the load wheel is initially advanced at full speed until its load cells detect an increase in the instantaneous load. When the measured load is within 200 pounds of a set point load, the analog velocity set point to the D.C. drive is reduced as the “load error” decreases. In this system, it is necessary to load to an average radial force, which is calculated over one or more revolutions of the tire. When the load is within the desired tolerance, the drive motor is disabled and a mechanical brake is engaged to maintain the final position of the load wheel.
In existing systems, the mechanical brake mechanism has been a critical component. During loading, the electrically released brake mechanism must fully release or else it can prevent proper loading and can cause excessive brake wear. At the end of the loading cycle, the brake release mechanism is de-energized and the brake must fully resist the back driving force generated by a loaded tire. Even a small amount of brake slippage results in degraded measurement results for the tire uniformity machine.